Error allocating memory of unsigned longs in C

I am writing a program that needs to use some pretty large arrays. I allocate memory for my array by calling:

unsigned long *visited = declareArrayofLongs(my_pow(2, d));

The declareArrayofLongs function is:

unsigned long int* declareArrayofLongs(unsigned long int length)
{
    unsigned long int *myarray = (unsigned long int*) malloc(length*sizeof(unsigned long int));
    if (myarray==NULL)
        printf("Error allocating memory for unsigned longs!\n");
    unsigned long int i;
    for(i=0; i<length; i++)
        myarray[i] = 0;
    return myarray;
}

And my_pow(2, d) returns 2^d as an unsigned long. When the program runs and gets up to around d=29, I get "Error allocating memory for unsigned longs!". I have 16gb of RAM on my system. Shouldn't I be able to handle this amount of allocation?

Only one visited array is declared at a time. I have a free(visited) call before re-allocating.